Focused ultrasonic therapy uses localized heating to destroy tumors or other tissue anomalies. Heating tissue beyond a critical temperature for a period of time causes the destruction of tissue (necrosis). Using Magnetic Resonance Imaging (MRI) guidance to guide the focal point of an ultrasonic therapy device is well known. For instance, U.S. Pat. Nos. 5,443,068, 5,275,165, and 5,247,935, each describes using an ultrasonic transducer guided by an MRI system to selectively destroy tissue.
In order to accurately position a focused ultrasonic therapy device, a positioner may be employed, which should provide repeatedly predictable control of the ultrasonic transducer. For example, tumors that are small or have irregular shapes require exact positioning of the ultrasonic transducer in order to destroy only the intended tissue while leaving the surrounding healthy tissue undamaged.
Known positioners, such as those described in U.S. Pat. Nos. 5,247,935 and 5,275,165, use hydraulic mechanisms to position an ultrasonic transducer beneath a patient. These systems have inherent reliability and accuracy problems due to the hydraulic positioners, which may experience motor backlash, degrading the accuracy of the positioner.
The need to accurately position an ultrasonic transducer for use in selective tissue necrosis presents additional problems when the transducer is used in combination with a Magnetic Resonance Imaging (MRI) guidance system. MRI systems employ large magnets for creating a homogenous magnetic field, and gradient coils for altering the magnetic field in a uniform manner in time and/or space to create magnetic field gradients. MRI systems also employ radio frequency (RF) coils for applying an RF field to the tissue that is to be imaged, causing the tissue to resonate and create an MR response signal. The MR response signal is then used to construct an image of the tissue that may be displayed, printed, and/or stored for later use and analysis. The degree of homogeneity of the magnetic field and the linearity of the magnetic field gradient over space and time are important in creating a clear undistorted image. Any interference with the RF field may reduce the quality of the image. The best and most consistent imaging typically occurs when surgical equipment or other objects do not interfere with the magnetic and RF fields created by the MRI system.
For example, equipment that is constructed from ferro-magnetic materials should not be used near an MRI system since the large magnetic fields generated by the MRI system may physically attract the magnetic equipment. Furthermore, conductive materials may disturb and distort the radio frequency electromagnetic fields necessary for resonance imaging. Other problems may occur with materials that produce eddy currents when placed in a time-varying magnetic field. The eddy currents in these materials, usually electrical conductors, may create their own magnetic field that may interfere with the fields used for magnetic resonance imaging. Therefore, materials that exhibit good conductivity, such as aluminum and copper, should not be used within a time-varying magnetic field.
For these reasons, motors of positioners used to move the transducer may be placed at a significant distance from the ultrasonic transducer and MRI system, i.e., outside the MRI imaging space. Such positioners therefore require long drive shafts and/or multiple joints, which may increase the physical footprint of the positioner. This arrangement also may cause inaccuracies in determining the actual position of the transducer due to mechanical freedom and elasticity of the transmission components extending from the motor to the ultrasonic transducer.
For example, U.S. Pat. No. 5,443,068 describes an MRI guided ultrasonic therapy system that uses threaded shafts attached to screw drives through universal joints in order to position a transducer in three orthogonal linear dimensions. The screw drives, and particularly the universal joints, used in this system compound motor backlash problems and therefore may limit the accuracy of the system. Furthermore, the motor drives may be formed from magnetic material and, therefore, are located away from the imaging space to eliminate interference with the MRI system. Therefore, this system may introduce reliability and accuracy problems explained above.
Accordingly, positioning systems for accurately positioning a therapeutic or diagnostic device, such as an ultrasound transducer, would be useful.